Hafnium complexes arene

Sixteen-electron bis(arene) complexes of formally zero-valent zirconium, hafnium, and titanium have been made by cocondensation of the metal atoms with bulky ligands such as 1,3,5-tri-t-butylbenzene71 ... 

Zero-valent zirconium and hafnium compounds remain relatively rare, owing to the strong thermodynamic driving force for the second and third row metals to attain a higher oxidation state. Despite this obstacle, examples of formally zero-valent compounds have been reported and characterized. The majority of these are arene complexes, whose syntheses and resulting chemistry have been reviewed.1,2 In addition to arene compounds, formally zero-valent butadiene complexes have also been described and are the subject of a rather comprehensive review.3 The focus of this section will be on compounds that have not been covered. 

The second strategy involves stabilization of the mono-Cp r/°-non-benzyl dialkyl metal cation with 7t-arene coordination. The arene coordination becomes the primary stabilization factor enabling the detection or isolation of the mono-Cp dimethyl cations. Thus, the reaction of Cp MMe3 (M = Zr, Hf) with 1 equiv. of B(C6Fs)3 in toluene/hexanes (1 10) solutions at ambient temperature affords the cationic arene complexes [(Gp M(Me)2(7]6-PhMe)][MeB(C6Fs)3] 304 as the solvent-separated ion pairs stabilized by the coordination of the aromatic solvent (Equation (22)).244 The crystal stmeture of the hafnium complex [(Cp Hf(Me)2(7]6-PhMe)][MeB(C6Fs)3] confirms the formation of the separated, discrete ion pairs in which the bent-sandwich cation is coordinated to an 7]6-toluene ligand.245... 

Triangular zirconium mixed-valence complexes stabilized by arenes, [Zr3(/u.-Cl)6( -C6Me6)3]"+ (n = 1, 2) see Arene Complexes, have been isolated as salts with the [Al2Cl7] anion (equation 28). Arenes also stabilize divalent hafnium in the complexes (/x-j -CeHsR)[Hfl2(PMe2Ph)2]2 (R = H, Me), which have a piano stool structure see Piano Stool Structure). ... 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

Bis(arene)hafnium complexes, characteristics, 4, 697 Bis(arene)iron dications, characteristics, 6, 173 Bis(arene)niobium complexes, characteristics, 5, 95 Bis(arene)titanium(0) complexes, characteristics, 4, 243-244 Bis(arene)tricarbonylchromium complex, synthesis, 5, 258... 

Incorporation of sterically demanding aryl substituents allows isolation of bis(arene)zirconium and hafnium complexes. The bond enthalpies of (7]6-(l,3,5-tBu)3C6H3)2M (M = Ti, 1 Zr, 2 Hf, 3) have been measured by iodinolytic bath calorimetry and values of 49(1), 64(3), and 67(4) kcal mol-1 have been determined for the respective metal-arene bond enthalpies (Scheme l).4 Computational studies establish that the major metal-arene bonding interaction is a 5-backbond formed from the overlap of metal dxz-yz and orbitals with the appropriate linear combination of arene p-orbitals. The observed increase in metal-arene bond strength is consistent with increased backbonding down the... 

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